Add condition that PRs are merged in backport workflow (#5997)

This brings our backport workflow script in line with the script from
the [tibdex/backport][1] github action implementation.

[1]: https://github.com/tibdex/backport/blob/b1bb2aaf611d9f2e5d1900c06493769b9f102a45/.github/workflows/backport.yml#L12-L22

Signed-off-by: Andrew Ross <andrross@amazon.com>

Author: andrross

.github/workflows/backport.yml

@@ -7,12 +7,22 @@ on:
 
 jobs:
   backport:
-    if: ${{ contains(github.event.label.name, 'backport') }}
+    name: Backport
     runs-on: ubuntu-latest
+    # Only react to merged PRs for security reasons.
+    # See https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target.
+    if: >
+      github.event.pull_request.merged
+      && (
+        github.event.action == 'closed'
+        || (
+          github.event.action == 'labeled'
+          && contains(github.event.label.name, 'backport')
+        )
+      )
     permissions:
       contents: write
       pull-requests: write
-    name: Backport
     steps:
       - name: GitHub App token
         id: github_app_token